The present invention relates to a method for making printable self-adhesive tapes, provided with a supporting member or film consisting of olefin polymers or copolymers, in particular polypropylene.
It is known that, a film of polypropylene or any other olefin polymer or copolymer is not receptive with respect to the printing inks and, accordingly, it cannot be printed.
It is also known that, in order to render the printing possible, it is necessary to surface process the film by electric charges, or by flame processing, in such a way as to modify the surface of the film, so as to provide the chemical-physical conditions suitable for receiving and holding inks.
This processing is carried out to increase the film surface tension up to 40 dynes/cm (on the not processed film it is of 30 dynes/cm), so as to provide a more wettable film with respect to the printing inks.
In fact, by means of the aforesaid processes, there is increased the contact angle between the film (which is solid) and the inks (which are liquid), in such a way as to facilitate the adhesion of the inks, according to the first adhesion law.
The practical measurement of the surface tension is carried out by means of known surface tension liquids, according to the methods of TAPPI and ASTM.
There are known in the trade printing inks suitable for printing processed polypropylene, which printing inks are conventionally used for printing materials for packaging applications and the like.
In the case of self-adhesive tapes provided with a polypropylene or other olefin polymer supporting film or member, as provided for printing applications, it is necessary to process the film on a surface, in such a way as to allow the film to receive the adhesive layer (either with or without an intermediate anchoring layer), and on the other surface, to receive the printed marks.
On the other hand, this two-step processing may cause the so-called blocking phenomena to occur, due to the strong adhesion of the self-adhesive layer on the back, with consequent breakage during the unwinding of the rolls.
In order to eliminate the abovementioned drawback, one may think to apply on the adhesive free surface an antiadhesive layer, the so-called "release coat." This antiadhesive layer, however, presents the drawback of being repulsive with respect to the printing inks.
In some cases it is possible to solve this problem by applying a lacquer on the back of the tape or by using, for making said tape, a mono- or bi-lacquer supporting member or film.
These lacquered supporting members are commercially available and used, mainly, in order to render the tape material thermosealable, in addition to being printable. However, apart the cost, the application of these lacquers (which generally are based on acrylic and vinyl copolymers) in making printable autoadhesive tapes, requires the use of a primer. Thus, the application of the lacquer has to be carried out by two steps.
This two step method, on the other hand, is different from the presently used method for making self-adhesive tapes provided with a polyethylene supporting member, in which all of the operations are carried out by a single step.
In fact, it is well-known that, the presently used method for making self-adhesive tapes provided with a processed polypropylene supporting member or film consists of applying, on a face of the film, an anchoring layer and an adhesive layer and, on the other face of the film, an antiadhesive layer.
More specifically, the anchoring layer, which, in some cases may also be omitted, generally consists of a solution or dispersion of elastomeric and/or plastics materials.
The adhesive layer, on the other hand, consists of a solution of elastomeric materials, modified by tackifying resins, in organic solvents, or of aqueous dispersions of elastomeric and/or plastics materials.
Alternatively, the layer may consist of adhesive materials based on block elastomeric copolymers, plastics materials (acrylic, polyesters, and so on), or of solvent free adhesives, based on acrylic prepolymers, polyurethane prepolymers, and so on which may be cross-linked by chemical methods or by radiation, such as U.V and E.B radiation.
The materials used to provide the antiadhesive layer consist of vinyl esters, long chain higher alcohols, including preferably from 14 to 22 carbon atoms, or polyvinyl carbamates, also of the long chain type.
These compounds, however as discussed hereinabove, are repulsive with respect to the printing inks, so that the obtained self-adhesive tapes can not be printed.